Power transmission system of hybrid electric vehicle

ABSTRACT

A power transmission system of a hybrid electric vehicle may include an input shaft receiving torque of an engine, an output gear outputting changed torque, a first planetary gear set disposed on the input shaft, and including a first rotation element connected to a first motor/generator, a second rotation element connected to the output gear, and a third rotation element directly connected to the input shaft, a second planetary gear set including a fourth rotation element connected to a second motor/generator and selectively connected to the third rotation element, a fifth rotation element connected to the second rotation element, a sixth rotation element selectively connected to a transmission housing, frictional elements selectively connecting two rotation elements among three rotation elements of the second planetary gear set, selectively connecting the third rotation element to the fourth rotation element, or selectively connecting the sixth rotation element to the transmission housing.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Korean Patent Application No.10-2013-0076705 filed on Jul. 1, 2013, the entire contents of which isincorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power transmission system of a hybridvehicle. More particularly, the present invention relates to a powertransmission system of a hybrid vehicle which minimizes impact due totorque change of a motor/generator when being converted into a hybridoperating mode and reduces electric load and capacity of themotor/generator by giving more importance on mechanical power deliverypath when splitting engine power.

2. Description of Related Art

Environmentally-friendly technique of vehicles is very importanttechnique on which survival of future motor industry is dependent.Vehicle makers are focusing on development of environmentally-friendlyvehicles so as to meet environment and fuel consumption regulations.

Therefore, the vehicle makers have been developing future vehicles suchas electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cellelectric vehicles (FCEV).

Since the future vehicles have technical restrictions such as weight andcost, the vehicle makers keep observation upon hybrid electric vehiclesfor meeting exhaust gas regulations and improving fuel consumptionperformance and are competing desperately to put the hybrid electricvehicles to practical use.

The hybrid electric vehicles are vehicles using more than two powersource, and gasoline engines or diesel engines using fossil fuel andmotor/generators driven by electrical energy are mainly used as thepower source of the hybrid electric vehicles.

The hybrid electric vehicle uses the motor/generator having relativelybetter low-speed torque characteristics as a main power source at alow-speed and uses an engine having relatively better high-speed torquecharacteristics as a main power source at a high-speed.

Since the hybrid electric vehicle stops operation of the engine usingthe fossil fuel and uses the motor/generator at a low-speed region, fuelconsumption may be improved and exhaust gas may be reduced.

The power transmission system of a hybrid electric vehicle is classifiedinto a single-mode type and a multi-mode type.

A torque delivery apparatus such as clutches and brakes for shiftcontrol is not necessary, but fuel consumption is high due todeterioration of efficiency at a high-speed region and an additionaltorque multiplication device is required for being applied to a largevehicle according to the single-mode type.

Since the multi-mode type has high efficiency at the high-speed regionand is able to multiply torque autonomously, the multi-mode type can beapplied to a full size vehicle.

Therefore, the multi-mode type instead of the single-mode type isapplied as the power transmission system of a hybrid electric vehicleand is also under continuous investigation.

The power transmission system of the multi-mode type includes aplurality of planetary gear sets, a plurality of motor/generatorsoperated as a motor and/or a generator, a plurality of torque deliveryapparatus controlling rotation elements of the planetary gear sets, anda battery used as a power source of the motor/generators.

The power transmission system of the multi-mode type has differentoperating mechanisms depend on connections of the planetary gear sets,the motor/generators, and the torque delivery apparatus.

In addition, the power transmission system of the multi-mode type hasdifferent features such a durability, power delivery efficiency, andsize depend on the connections of the planetary gear sets, themotor/generators, and the torque delivery apparatus. Therefore, designsfor the connection structure of the power transmission system of ahybrid electric vehicle are also under continuous investigation toachieve robust and compact power transmission system having no powerloss.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing apower transmission system of a hybrid electric vehicle having advantagesof minimizing impact due to torque change of a motor/generator whenbeing converted into a hybrid operating mode and reducing electric loadand capacity of the motor/generator by giving more importance onmechanical power delivery path when splitting engine power.

In addition, various aspects of the present invention are directed toproviding a power transmission system of a hybrid electric vehiclehaving further advantages of providing engine modes where a vehicle runswithout consuming electric energy at the motor/generator for enhancementof fuel consumption when high-speed traveling.

A power transmission system of a hybrid electric vehicle according to anaspect of the present invention may include, an input shaft receivingtorque of an engine. an output gear outputting changed torque. a firstplanetary gear set disposed on the input shaft, and including a firstrotation element connected to a first motor/generator, a second rotationelement connected to the output gear, and a third rotation elementdirectly connected to the input shaft. a second planetary gear setincluding a fourth rotation element connected to a secondmotor/generator and selectively connected to the third rotation element,a fifth rotation element connected to the second rotation element, asixth rotation element selectively connected to a transmission housing.frictional elements selectively connecting two rotation elements amongthree rotation elements of the second planetary gear set, selectivelyconnecting the third rotation element to the fourth rotation element, orselectively connecting the sixth rotation element to the transmissionhousing.

In various exemplary embodiments, the first planetary gear set may be asingle pinion planetary gear set, and may include a first sun gear beingthe first rotation element, a first planet carrier being the secondrotation element, and a first ring gear being the third rotationelement. In addition, the second planetary gear set may be a singlepinion planetary gear set, and may include a second sun gear being thefourth rotation element, a second planet carrier being the fifthrotation element, and a second ring gear being the sixth rotationelement.

In various exemplary embodiments, the first planetary gear set may be asingle pinion planetary gear set, and may include a first sun gear beingthe first rotation element, a first planet carrier being the secondrotation element, and a first ring gear being the third rotationelement. In addition, the second planetary gear set may be a doublepinion planetary gear set, and may include a second sun gear being thefourth rotation element, a second ring gear being the fifth rotationelement, and a second planet carrier being the sixth rotation element.

In various exemplary embodiments, the first planetary gear set may be adouble pinion planetary gear set, and may include a first sun gear beingthe first rotation element, a first ring gear being the second rotationelement, and a first planet carrier being the third rotation element. Inaddition, the second planetary gear set may be a single pinion planetarygear set, and may include a second sun gear being the fourth rotationelement, a second planet carrier being the fifth rotation element, and asecond ring gear being the sixth rotation element.

In various exemplary embodiments, the frictional elements may include, afirst clutch selectively connecting two rotation elements among threerotation elements of the second planetary gear set. a second clutchselectively connecting the third rotation element to the fourth rotationelement. a first brake selectively connecting the sixth rotation elementto the transmission housing.

In various exemplary embodiments, the first clutch may be disposedbetween the fourth rotation element and the sixth rotation element.

In various exemplary embodiments, the first clutch may be disposedbetween the fourth rotation element and the fifth rotation element.

In various exemplary embodiments, the first clutch may be disposedbetween the fifth rotation element and the sixth rotation element.

In various exemplary embodiments, the first brake may be operated at afirst EV mode, the first clutch may be operated at a second EV mode, thefirst brake may be operated at a first hybrid operating mode, the firstclutch may be operated at a second hybrid operating mode, the secondclutch may be operated at a third hybrid operating mode, the secondclutch and the first brake may be operated at a first engine mode, andthe first clutch and the second clutch may be operated at a secondengine mode.

In various exemplary embodiments, a third clutch may be disposed betweenthe engine and the input shaft.

A power transmission system of a hybrid electric vehicle according toanother aspect of the present invention may include, an input shaftreceiving torque of an engine. an output gear outputting changed torque,a first planetary gear set including a first rotation element connectedto a first motor/generator, a second rotation element connected to theoutput gear, and a third rotation element directly connected to theinput shaft, a second planetary gear set including a fourth rotationelement connected to a second motor/generator and selectively connectedto the third rotation element, a fifth rotation element connected to thesecond rotation element, a sixth rotation element selectively connectedto a transmission housing, a first clutch selectively connecting tworotation elements among three rotation elements of the second planetarygear set, a second clutch selectively connecting the third rotationelement to the fourth rotation element, a first brake selectivelyconnecting the sixth rotation element to the transmission housing.

In various exemplary embodiments, the first planetary gear set may be asingle pinion planetary gear set, and may include a first sun gear beingthe first rotation element, a first planet carrier being the secondrotation element, and a first ring gear being the third rotationelement. In addition, the second planetary gear set may be a singlepinion planetary gear set, and may include a second sun gear being thefourth rotation element, a second planet carrier being the fifthrotation element, and a second ring gear being the sixth rotationelement.

In various exemplary embodiments, the first planetary gear set may be asingle pinion planetary gear set, and may include a first sun gear beingthe first rotation element, a first planet carrier being the secondrotation element, and a first ring gear being the third rotationelement. In addition, the second planetary gear set may be a doublepinion planetary gear set, and may include a second sun gear being thefourth rotation element, a second ring gear being the fifth rotationelement, and a second planet carrier being the sixth rotation element.

In various exemplary embodiments, the first planetary gear set may be adouble pinion planetary gear set, and may include a first sun gear beingthe first rotation element, a first ring gear being the second rotationelement, and a first planet carrier being the third rotation element. Inaddition, the second planetary gear set may be a single pinion planetarygear set, and may include a second sun gear being the fourth rotationelement, a second planet carrier being the fifth rotation element, and asecond ring gear being the sixth rotation element.

In various exemplary embodiments, the first brake may be operated at afirst EV mode, the first clutch may be operated at a second EV mode, thefirst brake may be operated at a first hybrid operating mode, the firstclutch may be operated at a second hybrid operating mode, the secondclutch may be operated at a third hybrid operating mode, the secondclutch and the first brake may be operated at a first engine mode, andthe first clutch and the second clutch may be operated at a secondengine mode.

In various exemplary embodiments, the first clutch may be disposedbetween the fourth rotation element and the sixth rotation element.

In various exemplary embodiments, the first clutch may be disposedbetween the fourth rotation element and the fifth rotation element.

In various exemplary embodiments, the first clutch may be disposedbetween the fifth rotation element and the sixth rotation element.

In various exemplary embodiments, a third clutch may be disposed betweenthe engine and the input shaft.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a power transmission system accordingto the various exemplary embodiments of the present invention.

FIG. 2 is an operational chart of frictional elements at each modeapplied to a power transmission system according to the variousexemplary embodiments of the present invention.

FIG. 3 is a schematic diagram briefly illustrating connection ofrotation elements in a power transmission system according to thevarious exemplary embodiments of the present invention.

FIG. 4 to FIG. 7 are drawings for explaining operation of a powertransmission system according to the various exemplary embodiments ofthe present invention at each mode.

FIG. 8 is a schematic diagram of a power transmission system accordingto the various exemplary embodiments of the present invention.

FIG. 9 is a schematic diagram of a power transmission system accordingto the various exemplary embodiments of the present invention.

FIG. 10 is a schematic diagram of a power transmission system accordingto the various exemplary embodiments of the present invention.

FIG. 11 is a schematic diagram of a power transmission system accordingto the various exemplary embodiments of the present invention.

FIG. 12 is a schematic diagram of a power transmission system accordingto the various exemplary embodiments of the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the present invention as disclosed herein,including, for example, specific dimensions, orientations, locations,and shapes will be determined in part by the particular intendedapplication and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

Exemplary embodiments of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

Description of components that are not necessary for explaining thepresent exemplary embodiment will be omitted, and the same constituentelements are denoted by the same reference numerals in thisspecification.

In the detailed description, ordinal numbers are used for distinguishingconstituent elements having the same terms, and have no specificmeanings.

FIG. 1 is a schematic diagram of a power transmission system accordingto the first exemplary embodiment of the present invention.

Referring to FIG. 1, a power transmission system of a hybrid electricvehicle according to the first exemplary embodiment of the presentinvention changes torque of an engine ENG transmitted through an inputshaft IS according to running state of a vehicle, and outputs thechanged torque through an output gear OG.

The power transmission system includes first and second planetary gearsets PG1 and PG2, first and second motor/generators MG1 and MG2, andfrictional elements including first and second clutches CL1 and CL2 anda first brake BK1.

One rotation element of the first planetary gear set PG1 is directlyconnected to one rotation element of the second planetary gear set PG2and another rotation element of the first planetary gear set PG1 isselectively connected to another rotation element of the secondplanetary gear set PG2 such that the first planetary gear set PG1 andthe second planetary gear set PG2 are operated as one compound planetarygear set.

The first motor/generator MG1 and the second motor/generator MG2 areindependent power sources and are operated as a motor and a generator.

The first motor/generator MG1 is operated as a motor that suppliestorque to the other rotation element of the first planetary gear set PG1or is operated as a generator that generates electricity by torque ofthe other rotation element.

The second motor/generator MG2 is operated as a motor that suppliesanother rotation element of the second planetary gear set PG2 or isoperated as a generator that generates electricity by torque of anotherrotation element.

For this purpose, a stator of the first motor/generator MG1 and a statorof the second motor/generator MG2 are fixed to a transmission housing H,and a rotor of the first motor/generator MG1 and a rotor of the secondmotor/generator MG2 are connected respectively to the other rotationelement of the first planetary gear set PG1 and another rotation elementof the second planetary gear set PG2.

The first clutch CL1 is operated as a selective direct-coupling means.The first clutch CL1 connects two rotation elements among three rotationelements of the second planetary gear set PG2 so as to directly couplethe second planetary gear set PG2.

The second clutch CL2 and the first brake BK1 are operated as selectiveconnecting means. The second clutch CL2 selectively connects anotherrotation element of the first planetary gear set PG1 to another rotationelement of the second planetary gear set PG2, and the first brake BK1selectively connects the other rotation element of the second planetarygear set PG2 to the transmission housing H.

The first and second clutches CL1 and CL2 are frictional elementsselectively connecting a rotation element with another rotation element,and the first brake BK1 is a frictional element selectively connecting arotation element with a fixed element (i.e., transmission housing). Thefirst and second clutches CL1 and CL2 and the brake BK may beconventional multi-plate friction elements of wet type that are operatedby hydraulic pressure.

The power transmission system of a hybrid electric vehicle according tothe first exemplary embodiment of the present invention will bedescribed in further detail.

The first planetary gear set PG1 is a single pinion planetary gear set,and includes the first rotation element N1 being a first sun gear S1,the second rotation element N2 being a first planet carrier PC1rotatably supporting a first pinion P1 externally meshed with the firstsun gear S1, and the third rotation element N3 being a first ring gearR1 internally meshed with the first pinion P1.

The second planetary gear set PG2 is a single pinion planetary gear set,and includes the fourth rotation element N4 being a second sun gear S2,the fifth rotation element N5 being a second planet carrier PC2rotatably supporting a second pinion P2 externally meshed with thesecond sun gear S2, and the sixth rotation element N6 being a secondring gear R2 internally meshed with the second pinion P2.

The first planet carrier PC1 of the second rotation element N2 isdirectly connected to the second planet carrier PC2 of the fifthrotation element N5. The second rotation element N2 and the fifthrotation element N5 are directly connected to the output gear OG, andthe first ring gear R1 of the third rotation element N3 is selectivelyconnected to the second sun gear S2 of the fourth rotation element N4and is directly connected to the input shaft IS.

The input shaft IS receives torque from the engine ENG, and the outputgear OG is an output member and transmits driving torque to a drivingwheel through a final reduction gear and a differential apparatus.

The first motor/generator MG1 is directly connected to the first sungear S1 of the first rotation element N1 so as to be operated as themotor driving the first sun gear S1 or as the generator generatingelectricity by torque of the first sun gear S1.

The second motor/generator MG2 is directly connected to the second sungear S2 of the fourth rotation element N4 so as to be operated as themotor driving the second sun gear S2 or as the generator generatingelectricity by torque of the second sun gear S2.

The first clutch CL1 is disposed between the second sun gear S2 of thefourth rotation element N4 and the second ring gear R2 of the sixthrotation element N6 so as to selectively connect the second sun gear S2to the second ring gear R2.

The second clutch CL2 selectively connects the second sun gear S2 of thefourth rotation element N4 to the input shaft IS and the first ring gearR1 of the third rotation element N3.

The first brake BK1 is disposed between the second ring gear R2 of thesixth rotation element N6 and the transmission housing H so as toselectively connect the second ring gear R2 to the transmission housingH.

FIG. 2 is an operational chart of frictional elements at each modeapplied to a power transmission system according to the first exemplaryembodiment of the present invention.

Referring to FIG. 2, a first EV (Electric Vehicle) mode is achieved byoperating the first brake BK1.

A second EV mode is achieved by operating the first clutch CL1.

A first hybrid operating mode is achieved by operating the first brakeBK1.

A second hybrid operating mode is achieved by operating the first clutchCL1.

A third hybrid operating mode is achieved by operating the second clutchCL2.

A first engine mode is achieved by operating the second clutch CL2 andthe first brake BK1.

A second engine mode is achieved by operating the first clutch CL1 andthe second clutch CL2.

As described above, the power transmission system according to the firstexemplary embodiment of the present invention can realize two EV modes,three hybrid operating modes, and two engine modes.

FIG. 3 is a schematic diagram briefly illustrating connection ofrotation elements in a power transmission system according to the firstexemplary embodiment of the present invention.

Referring to FIG. 3, the second rotation element N2 and the fifthrotation element N5 are directly connected and the third rotationelement N3 and the fourth rotation element N4 are selectively connectedthrough the second clutch CL2 in the power transmission system accordingto the first exemplary embodiment of the present invention.

In addition, the first rotation element N1 is directly connected to thefirst motor/generator MG1, the third rotation element N3 is connected tothe engine ENG, and the fourth rotation element N4 is directly connectedto the second motor/generator MG2.

In addition, the sixth rotation element N6 is selectively connected tothe transmission housing H through the first brake BK1 and isselectively connected to the fourth rotation element N4 through thefirst clutch CL1.

FIG. 4 to FIG. 7 are drawings for explaining operation of a powertransmission system according to the first exemplary embodiment of thepresent invention at each mode. FIG. 4 to FIG. 7 illustrate change ofinput rotation speed assuming that output rotation speed is constant.

[First EV Mode]

FIG. 4A is a drawing for explaining operation of the power transmissionsystem according to the first exemplary embodiment of the presentinvention at the first EV mode.

The EV mode is a mode where power of a battery is supplied to amotor/generator in a stopped state of an engine such that a vehicle isdriven by power of the motor/generator.

Since the engine is stopped, fuel economy may be enhanced, and thevehicle can move rearward without an additional reverse speed device atthe EV mode. The EV mode is utilized when the vehicle is started in astopped state or the vehicle runs with a low speed. A reduced gear ratiowhere the power source rotates faster than an output member is requiredfor assisting hill-start or quick acceleration.

Under such conditions, the sixth rotation element N6 is operated as afixed element by operation of the first brake BK1, and the secondmotor/generator MG2 is operated so as to input torque of the secondmotor/generator MG2 to the fourth rotation element N4 at the first EVmode. Therefore, the torque of the second motor/generator MG2 is changedaccording to the gear ratio of the second planetary gear set PG2 and thechanged torque is output.

At this time, the first planetary gear set PG1 is not involved in ashift. However, the third rotation element N3 is stopped together withthe engine and the second rotation element N2 is directly connected tothe fifth rotation element N5. Therefore, the first and second rotationelements N1 and N2 are idling.

[Second EV Mode]

FIG. 4B is a drawing for explaining operation of the power transmissionsystem according to the first exemplary embodiment of the presentinvention at the second EV mode.

Efficiency of the motor/generator changes according to rotation speedand torque thereof. This means that a conversion ratio of electricalenergy into mechanical energy is different according to the rotationspeed and the torque of the motor/generator even though the same amountof current is supplied.

Current of the battery used at EV mode is generated by burning fuel inthe engine or being accumulated by regenerative braking, but how to usethe accumulated energy efficiently affects on enhancement of fueleconomy directly.

For this reason, a transmission realizing more than two shift-speeds isincreasingly mounted on the electric vehicle, and it is advantageousthat the hybrid vehicle realizes the EV mode having more than twoshift-speeds. Therefore, the exemplary embodiment of the presentinvention can realize two EV modes.

Shifting processes to the second EV mode are as follows. When vehiclespeed increases during the vehicle drives at the first EV mode,efficiency of the second motor/generator MG2 is deteriorated. At thistime, when the first brake BK1 is released and the first clutch CL1 isoperated at a point where the efficiency of the second motor/generatorMG2 is bad, the second EV mode is achieved.

In this case, since the first clutch CL1 that is the direct-couplingdevice of the second planetary gear set PG2 is operated, the secondplanetary gear set PG2 becomes a direct-coupling state. Therefore, allthe rotation elements N4, N5, and N6 of the second planetary gear setPG2 rotate with the same speed and the driving torque is output throughthe fifth rotation element N5.

At this time, the first planetary gear set PG1 is not involved in ashift. However, the third rotation element N3 is stopped together withthe engine, and the first and second rotation elements N1 and N2 areidling.

[First Hybrid Operating Mode]

FIG. 5A is a drawing for explaining operation of the power transmissionsystem according to the first exemplary embodiment of the presentinvention at the first hybrid operating mode.

The torque of the engine is delivered to the output member throughmechanical path and electrical path at the first hybrid operating mode,and such split of the engine torque is done by the planetary gear set.Since the engine and the motor/generator connected to the planetary gearset can control their rotation speeds regardless of the vehicle speed,the power transmission system at the first hybrid operating mode isoperated as an electric continuously variable transmission.

The speed and the torque of the engine are fixed at given vehicle speedin a conventional transmission, but the speed and the torque of theengine can be changed freely at the given vehicle speed in the electriccontinuously variable transmission. Therefore, driving efficiency of theengine may be maximized and fuel economy may be enhanced.

Shifting processes to the first hybrid operating mode are as follows.The second rotation element N2 of the first planetary gear set PG1 aswell as the fifth rotation element N5 is connected to the output gearOG, but the first and third rotation elements N1 and N3 can rotatefreely at the first EV mode (referring to FIG. 4A).

If the engine ENG is started using the first motor generator MG1 at thisstate, speeds of the engine ENG and the first motor/generator MG1 can becontrolled regardless of the vehicle speed.

Therefore, when the engine ENG and the first motor/generator MG1 arecontrolled, the torque of the engine and the torque of the firstmotor/generator MG1 are summed up and the summed torque is delivered tothe fifth rotation element N5 that is the output element. Therefore,high driving torque may be generated.

At this time, as shown in FIG. 5A, the first motor/generator MG1 isoperated as the generator when rotating negatively, and the firstmotor/generator MG1 is operated as the motor when rotating positively(at this time, the rotation speed of the engine ENG decreases).

In addition, the sixth rotation element N6 is operated as the fixedelement by operation of the first brake BK1, the driving torque isoutput through the fifth rotation element N5, and the fourth rotationelement N4 is idling in the second planetary gear set PG2.

Since the engine ENG and the first motor/generator MG1 can be controlledindependently at the first hybrid operating mode, fuel economy anddriving performance may be greatly enhanced.

[Second Hybrid Operating Mode]

FIG. 5B is a drawing for explaining operation of the power transmissionsystem according to the first exemplary embodiment of the presentinvention at the second hybrid operating mode.

If the vehicle speed increased during the vehicle runs at the firsthybrid operating mode, the first brake BK1 is released and the firstclutch CL1 is operated so as to lower rotation speeds of all therotation elements of the first planetary gear set PG1. Accordingly, thesecond hybrid operating mode begins.

Since the first clutch CL1 that is the direct-coupling device of thesecond planetary gear set PG2 is operated, the second planetary gear setPG2 becomes a direct-coupling state. Therefore, all the rotationelements N4, N5, and N6 of the second planetary gear set PG2 rotate withthe same speed.

Since the engine ENG and the first motor/generator MG1 can be controlledindependently at the second hybrid operating mode, the same as at thefirst hybrid operating mode, fuel economy and driving performance may begreatly enhanced.

[Third Hybrid Operating Mode]

FIG. 6 is a drawing for explaining operation of the power transmissionsystem according to the first exemplary embodiment of the presentinvention at the third hybrid operating mode.

Since the rotation speed of the motor/generator connected to the outputelement is restricted to the vehicle speed, the motor/generator cannotbe operated efficiently and capacity of the motor/generator is hard tobe reduced at the third hybrid operating mode.

Particularly, when the rotation speed of the motor/generator restrictedto the vehicle speed is high due to high vehicle speed, efficiency ofthe motor/generator may be deteriorated and fuel economy may be hard tobe optimized.

Under such conditions, one rotation element of the first planetary gearset PG1 and one rotation element of the second planetary gear set PG2connected to the engine ENG are coupled to another rotation element ofthe first planetary gear set PG1 and another rotation element of thesecond planetary gear set PG2 connected to the output gear OG so thatthe rotation speed of the engine ENG and the rotation speeds of twomotor/generators MG1 and MG2 are controlled regardless the vehiclespeed. Thereby, the power transmission system may be operated as acontinuously variable transmission and may enhance fuel economy.

If the second clutch CL2 is operated, the third rotation element N3 andthe fourth rotation element N4 are connected and rotated with the samespeed. In addition, since the second rotation element N2 and the fifthrotation element N5 are directly connected, the first planetary gear setPG1 and the second planetary gear set PG2 are integrally rotated androtation speeds and torques of the rotation elements are restricted byeach other.

In addition, since electrical energy of the first and secondmotor/generators MG1 and MG2 should be balanced, the speeds and thetorques of all the rotation elements of the first and second planetarygear sets PG1 and PG2 are correlated to each other and the powertransmission system operates as the electric continuously variabletransmission at the third hybrid operating mode.

The first hybrid operating mode and the second hybrid operating mode canbe converted into the third hybrid operating mode. That is, When thefirst hybrid operating mode and the second hybrid operating mode areconverted into the third hybrid operating mode, the second clutch CL2 isoperated. After that, the engine ENG and the second motor/generator MG2that can be independently controlled at the first and second hybridoperating modes are synchronized and the first brake BK1 or the firstclutch CL1 is released. Therefore, conversion to the third hybridoperating mode can be done smoothly.

That is, when the first hybrid operating mode is converted into thethird hybrid operating mode, the engine ENG and the secondmotor/generator MG2 are synchronized by operating the second clutch CL2and the first brake BK1 is released. When the second hybrid operatingmode is converted into the third hybrid operating mode, the engine ENGand the second motor/generator MG2 are synchronized by operating thesecond clutch CL2 and the first clutch CL1 is released.

When converting from the first hybrid operating mode or the secondhybrid operating mode to the third hybrid operating mode, direction oftorque of the first and second motor/generators MG1 and MG2 does notchange. Therefore, impact due to mode conversion does not occur.

[First Engine Mode]

FIG. 7A is a drawing for explaining operation of the power transmissionsystem according to the first exemplary embodiment of the presentinvention at the first engine mode.

Important technique in the hybrid vehicle for enhancing fuel economy isrecovery and reuse of brake energy and control of driving point of theengine.

In addition, controlling of the driving point of the engine accompaniesconversion of mechanical energy of the engine into electric energy ofthe motor/generator and conversion of the electric energy of themotor/generator back into mechanical energy at the motor/generator.

During energy conversion, all input energy is not output and energy lossoccurs. Since fuel economy at the engine mode where the vehicle isdriven only by the engine may be superior to that at the hybrid mode atany driving condition, the exemplary embodiments of the presentinvention provide two engine modes.

That is, the second clutch CL2 and the first brake BK1 are engaged atthe first engine mode. In this case, the third rotation element N3 andthe fourth rotation element N4 are rotated with the same speed as theengine, and the sixth rotation element N6 is operated as the fixedelement. Therefore, reduced speed ratio is generated.

At this time, since the first and second motor/generators MG1 and MG2 donot need to provide torque, the first engine mode at which the vehicleis driven only by the engine ENG is achieved.

[Second Engine Mode]

FIG. 7B is a drawing for explaining operation of the power transmissionsystem according to the first exemplary embodiment of the presentinvention at the second engine mode.

If the vehicle speed increases during the vehicle runs at the firstengine mode, the first brake BK1 is released and the first clutch CL1 isoperated.

In this case, all the rotation elements of the first and secondplanetary gear sets PG1 and PG2 rotate with the same speed by operationof the first and second clutches CL1 and CL2. Therefore, the torqueinput from the engine ENG is output without toque conversion.

Since the first and second motor/generators MG1 and MG2 need not toprovide torque at this time, the second engine mode at which the vehicleis driven only by the torque of the engine ENG is achieved.

According to the first exemplary embodiment of the present invention,two EV modes, three hybrid operating modes, and two engine modes can beachieved by combining two planetary gear sets PG1 and PG2, threefrictional elements CL1, CL2, and BK1, and two motor/generators MG1 andMG2.

In addition, the electric load may be reduced and the maximum power ofthe engine may be used by giving more importance on mechanical powerdelivery path when splitting the engine power. Further, the number ofmode conversions may be decreased when starting, and change in rotationspeed of all the rotation elements may be minimized when convertingmodes.

Since sufficient power performance is provided in WOT (Wide OpenThrottle) start, conversion to the engine mode is suppressed and maximumpower of the engine can be used in conversion among the first, second,and third hybrid operating modes.

In addition, the second clutch CL2 is disposed between the thirdrotation element N3 of the first planetary gear set PG1 and the fourthrotation element N4 of the second planetary gear set PG2 so as to freelyperform a mode changed from the first hybrid operating mode or thesecond hybrid operating mode into the third hybrid operating modeaccording to the first exemplary embodiment of the present invention.When the second clutch CL2 is operated, the engine ENG and the secondmotor/generator MG2 that can be independently controlled at the firstand second hybrid operating modes are synchronized and conversion to thethird hybrid operating mode is then achieved. Therefore, mode conversioncan be achieved smoothly.

Therefore, impact due to engagement of the second clutch CL2 may besuppressed and direction of torque of the first motor/generator MG1 andthe second motor/generator MG2 is maintained before or after modeconversion. Therefore, controllability may be excellent.

After mode conversion is achieved, the engine is operated at its maximumpower point at the third hybrid operating mode, and the rotation speedof the second motor/generator MG2 decreases as the vehicle speedincreases. Therefore, the vehicle can be driven within a drive limitrange of the motor.

Since the engine mode is provided, the vehicle can run with high speedwithout electric load of the first and second motor/generators MG1 andMG2. Therefore, fuel economy may be improved.

FIG. 8 is a schematic diagram of a power transmission system accordingto the second exemplary embodiment of the present invention.

Referring to FIG. 8, the first planetary gear set PG1 is the singlepinion planetary gear set in the first exemplary embodiment, but thefirst planetary gear set PG1 is a double pinion planetary gear set inthe second exemplary embodiment.

Therefore, the first sun gear S1 is the first rotation element N1, thefirst ring gear R1 is the second rotation element N2, and the firstplanet carrier PC1 is the third rotation element N3.

Since functions of the second exemplary embodiment are the same as thoseof the first exemplary embodiment except the rotation elements includingthe second and third rotation elements N2 and N3, detailed descriptionthereof will be omitted.

FIG. 9 is a schematic diagram of a power transmission system accordingto the third exemplary embodiment of the present invention.

Referring to FIG. 9, the second planetary gear set PG2 is the singlepinion planetary gear set in the first exemplary embodiment, but thesecond planetary gear set PG2 is a double pinion planetary gear set inthe third exemplary embodiment.

Therefore, the second sun gear S2 is the fourth rotation element N4, thesecond ring gear R2 is the fifth rotation element N2, and the secondplanet carrier PC2 is the sixth rotation element N6.

Since functions of the second exemplary embodiment are the same as thoseof the first exemplary embodiment except the rotation elements includingthe fifth and sixth rotation elements N5 and N6, detailed descriptionthereof will be omitted.

FIG. 10 is a schematic diagram of a power transmission system accordingto the fourth exemplary embodiment of the present invention.

Referring to FIG. 10, the first clutch CL1 that is direct-coupling meansof the second planetary gear set PG2 is disposed between the fourthrotation element N4 and the sixth rotation element N6 in the firstexemplary embodiment, but the first clutch CL1 is disposed between thefourth rotation element N4 and the fifth rotation element N5 in thefourth exemplary embodiment.

Since functions of the fourth exemplary embodiment are the same as thoseof the first exemplary embodiment except the position of the firstclutch CL1, detailed description thereof will be omitted.

FIG. 11 is a schematic diagram of a power transmission system accordingto the fifth exemplary embodiment of the present invention.

Referring to FIG. 11, the first clutch CL1 that is direct-coupling meansof the second planetary gear set PG2 is disposed between the fourthrotation element N4 and the sixth rotation element N6 in the firstexemplary embodiment, the first clutch CL1 is disposed between the fifthrotation element N5 and the sixth rotation element N6 in the fifthexemplary embodiment.

Since functions of the fifth exemplary embodiment are the same as thoseof the first exemplary embodiment except the position of the firstclutch CL1, detailed description thereof will be omitted.

FIG. 12 is a schematic diagram of a power transmission system accordingto the sixth exemplary embodiment of the present invention.

Referring to FIG. 12, a clutch that is selective power delivery means isnot disposed between the engine ENG and the third rotation element N3 inthe first exemplary embodiment, but a third clutch CL3 that is selectivepower delivery means is disposed between the engine ENG and the inputshaft IS in the sixth exemplary embodiment.

Therefore, the third clutch CL3 is operated at a mode where the torqueof the engine ENG is demanded, the torque of the engine ENG istransmitted to the input shaft IS.

Since functions of the sixth exemplary embodiment are the same as thoseof the first exemplary embodiment except whether the third clutch CL3exists or not, detailed description thereof will be omitted.

Two EV modes, three hybrid operating modes, and two engine modes areachieved by combining two planetary gear sets, three frictionalelements, and two motor/generators according to the exemplaryembodiments of the present invention.

In addition, the electric load may be reduced and the maximum power ofthe engine may be used by giving more importance on mechanical powerdelivery path when splitting the engine power. Further, the number ofmode conversions may be decreased when starting, and change in rotationspeed of all the rotation elements may be minimized when convertingmodes.

Since sufficient power performance is provided in WOT (Wide OpenThrottle) start, conversion to the engine mode is suppressed and maximumpower of the engine can be used in conversion between the hybrid inputsplit mode and the compound split mode.

In addition, the second clutch is disposed between the third rotationelement of the first planetary gear set and the fourth rotation elementof the second planetary gear set so as to freely perform a mode changedfrom the first hybrid operating mode or the second hybrid operating modeinto the third hybrid operating mode according to the exemplaryembodiments of the present invention. When the second clutch isoperated, the engine and the second motor/generator that can beindependently controlled at the first and second hybrid operating modesare synchronized and conversion to the third hybrid operating mode isthen achieved. Therefore, mode conversion can be achieved smoothly.

Therefore, impact due to engagement of the second clutch may besuppressed and direction of torque of the first motor/generator and thesecond motor/generator is maintained before or after mode conversion.Therefore, controllability may be excellent.

After mode conversion is achieved, the engine is operated at its maximumpower point at the third hybrid operating mode, and the rotation speedof the second motor/generator decreases as the vehicle speed increases.Therefore, the vehicle can be driven within a drive limit range of themotor.

Since the engine mode is provided, the vehicle can run with high speedwithout electric load of the first and second motor/generators.Therefore, fuel economy may be improved.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings as well as various alternatives and modifications thereof. Itis intended that the scope of the invention be defined by the Claimsappended hereto and their equivalents.

What is claimed is:
 1. A power transmission system of a hybrid electricvehicle comprising: an input shaft receiving torque of an engine; anoutput gear outputting changed torque; a first planetary gear setdisposed on the input shaft, and including: a first rotation elementconnected to a first motor and generator; a second rotation elementconnected to the output gear; and a third rotation element directlyconnected to the input shaft; a second planetary gear set including: afourth rotation element connected to a second motor and generator andselectively connected to the third rotation element or the input shaft;a fifth rotation element connected to the second rotation element; asixth rotation element selectively connected to a transmission housing;and frictional elements, wherein the frictional elements comprise: afirst clutch selectively connecting two rotation elements among thethree rotation elements of the second planetary gear set; a secondclutch selectively connecting the fourth rotation element to the thirdrotation element or the input shaft; and a first brake selectivelyconnecting the sixth rotation element to the transmission housing. 2.The power transmission system of claim 1, wherein the first planetarygear set is a single pinion planetary gear set, and includes: a firstsun gear being the first rotation element; a first planet carrier beingthe second rotation element; and a first ring gear being the thirdrotation element, and wherein the second planetary gear set is a singlepinion planetary gear set, and includes: a second sun gear being thefourth rotation element; a second planet carrier being the fifthrotation element; and a second ring gear being the sixth rotationelement.
 3. The power transmission system of claim 1, wherein the firstplanetary gear set is a single pinion planetary gear set, and includes:a first sun gear being the first rotation element; a first planetcarrier being the second rotation element; and a first ring gear beingthe third rotation element, and wherein the second planetary gear set isa double pinion planetary gear set, and includes: a second sun gearbeing the fourth rotation element; a second ring gear being the fifthrotation element; and a second planet carrier being the sixth rotationelement.
 4. The power transmission system of claim 1, wherein the firstplanetary gear set is a double pinion planetary gear set, and includes:a first sun gear being the first rotation element; a first ring gearbeing the second rotation element; and a first planet carrier being thethird rotation element, and wherein the second planetary gear set is asingle pinion planetary gear set, and includes: a second sun gear beingthe fourth rotation element; a second planet carrier being the fifthrotation element; and a second ring gear being the sixth rotationelement.
 5. The power transmission system of claim 1, wherein the firstclutch is disposed between the fourth rotation element and the sixthrotation element.
 6. The power transmission system of claim 1, whereinthe first clutch is disposed between the fourth rotation element and thefifth rotation element.
 7. The power transmission system of claim 1,wherein the first clutch is disposed between the fifth rotation elementand the sixth rotation element.
 8. The power transmission of claim 1,wherein the first brake is operated at a first Electric Vehicle (EV)mode; the first clutch is operated at a second Electric Vehicle (EV)mode; the first brake is operated at a first hybrid operating mode; thefirst clutch is operated at a second hybrid operating mode; the secondclutch is operated at a third hybrid operating mode; the second clutchand the first brake are operated at a first engine mode; and the firstclutch and the second clutch are operated at a second engine mode. 9.The power transmission system of claim 1, wherein a third clutch isdisposed between the engine and the input shaft.
 10. A powertransmission system of a hybrid electric vehicle comprising: an inputshaft receiving torque of an engine; an output gear outputting changedtorque; a first planetary gear set including: a first rotation elementconnected to a first motor and generator; a second rotation elementconnected to the output gear; and a third rotation element directlyconnected to the input shaft; a second planetary gear set including: afourth rotation element connected to a second motor and generator andselectively connected to the third rotation element or the input shaft;a fifth rotation element connected to the second rotation element; and asixth rotation element selectively connected to a transmission housing;and a first clutch selectively connecting two rotation elements amongthe three rotation elements of the second planetary gear set; a secondclutch selectively connecting the third rotation element to the fourthrotation element; and a first brake selectively connecting the sixthrotation element to the transmission housing.
 11. The power transmissionsystem of claim 10, wherein the first planetary gear set is a singlepinion planetary gear set, and includes: a first sun gear being thefirst rotation element; a first planet carrier being the second rotationelement; and a first ring gear being the third rotation element, andwherein the second planetary gear set is a single pinion planetary gearset, and includes: a second sun gear being the fourth rotation element;a second planet carrier being the fifth rotation element; and a secondring gear being the sixth rotation element.
 12. The power transmissionsystem of claim 10, wherein the first planetary gear set is a singlepinion planetary gear set, and includes: a first sun gear being thefirst rotation element; a first planet carrier being the second rotationelement; and a first ring gear being the third rotation element, andwherein the second planetary gear set is a double pinion planetary gearset, and includes: a second sun gear being the fourth rotation element;a second ring gear being the fifth rotation element; and a second planetcarrier being the sixth rotation element.
 13. The power transmissionsystem of claim 10, wherein the first planetary gear set is a doublepinion planetary gear set, and includes: a first sun gear being thefirst rotation element; a first ring gear being the second rotationelement; and a first planet carrier being the third rotation element,and wherein the second planetary gear set is a single pinion planetarygear set, and includes: a second sun gear being the fourth rotationelement; a second planet carrier being the fifth rotation element; and asecond ring gear being the sixth rotation element.
 14. The powertransmission of claim 10, wherein the first brake is operated at a firstElectric Vehicle (EV) mode; the first clutch is operated at a secondElectric Vehicle (EV) mode; the first brake is operated at a firsthybrid operating mode; the first clutch is operated at a second hybridoperating mode; the second clutch is operated at a third hybridoperating mode; the second clutch and the first brake are operated at afirst engine mode; and the first clutch and the second clutch areoperated at a second engine mode.
 15. The power transmission system ofclaim 10, wherein the first clutch is disposed between the fourthrotation element and the sixth rotation element.
 16. The powertransmission system of claim 10, wherein the first clutch is disposedbetween the fourth rotation element and the fifth rotation element. 17.The power transmission system of claim 10, wherein the first clutch isdisposed between the fifth rotation element and the sixth rotationelement.
 18. The power transmission system of claim 10, wherein a thirdclutch is disposed between the engine and the input shaft.